1. Field of the Invention
The present invention relates to an integrator, and in particular to an improved integrator input circuit which is capable of adjusting a frequency bandwidth of an integrator by dividing a current at a predetermined ratio when converting an input voltage into a current and inputting a part of the current into the integrator.
2. Description of the Background Art
FIG. 1 illustrates a known integrator. As shown therein, the known integrator includes a computation amplifier "A" for amplifying an input voltage value, a feeding-back capacitor "C" connected between an input terminal and an output terminal of the computation amplifier "A", and a resistor "R" connected between the voltage input terminal and the computation amplifier "A".
FIG. 2 illustrates a frequency response characteristic of the integrator. The operation characteristic of the integrator will be explained with reference to FIG. 2.
First, the input current flowing from the input terminal of the integrator, to which an input voltage Vs is applied, to the resistor "R" may be expressed as i.sub.1 =V.sub.S /R. In addition, the current flowing from the output terminal of the computation amplifier "A" to the capacitor "C" may be expressed as i.sub.c =C(dV.sub.0 /dt). Here, since current does not flow in the input terminal of the computation amplifier "A", an expression of i.sub.1 =-i.sub.C is obtained.
Therefore, the following equation is obtained. ##EQU1##
The output voltage V.sub.0 may be expressed as follows by integrating the values of Equation 1. ##EQU2##
Namely, the output voltage (V.sub.0) from the integrator is changed to a type of a mathematical integration with respect to the input voltage (Vs) based on Equation 2.
In the known integrator, the integration coefficient is 1/RC, and a frequency bandwidth which is one of the major characteristics of the integrator, as shown in FIG. 2 is reverse proportional to the capacity of the resistor R or the capacitor C.
Therefore, in order to obtain a predetermined frequency bandwidth, the capacity of the resistor R or the capacitor C is adjusted. If a predetermined frequency bandwidth is required, the same is obtained by properly adjusting the capacity of the resistor R or the capacitor C. If a very low frequency bandwidth is required, the capacity of the resistor R or the capacitor C should be very large.
However, if the resistance value is too increased, the resistance with respect to noise is decreased, so that there is a limit for implementing an integrated circuit (IC). In addition, it is difficult to increase the capacitance in the IC, so that a capacitor and an external terminal are additionally used.